The art of dry toner imaging is presently well developed and provides the basis of most office copying and printing systems in use today. The electrophotographic process is the most prevalent method of dry toner imaging. Electrophotographic copiers and printers normally employ five steps in the imaging process. The individual steps of the electrophotographic process include the following: (1) a uniform electric charge is deposited on a photoconductor drum or belt in the dark; (2) an electrostatic latent image is then created on the photoconductor by exposing the photoconductor to a pattern of light; (3) the photoconductor is then exposed to toner particles, wherein toner particles having the correct polarity are adhered to the latent image area; (4) a medium to be printed is then passed between the photoconductor and a transfer corona to cause the toner particles to transfer from the photoconductor to the medium; and (5) the transferred toner particles are then fixed to the medium by one of various procedures known in the art.
One important use of electrophotographic copiers and printers involves their use in making overhead projection transparencies. When preparing transparencies on electrophotographic copiers and printers, it is not only important that the transparent receptor film media reliably feed through the electrophotographic copying and printing machine utilized, but also that the receptive film media be able to provide good image quality and toner adhesion, while at the same time being resistant to hot fuser roll embossing to give a clear background.
There are many makes and models of copiers and printers on the market, which can produce images on opaque or transparent film substrates, utilizing electrophotographic processes, such as outlined above. During such electrophotographic processes, it is required that an image be fixed to the film substrate (see step 5 above). In such a fixing step, fixation of the image toner generally occurs by applying heat and pressure to rollers between which the imaged medium must pass, so as to bond the image to the film medium utilized. For higher speed copiers and printers, the speed with which the fixing step is completed often results in poor toner or image adhesion. On the other hand, with low speed hot fusing copiers and printers, low speeds can provide good image adhesion, but can also emboss the surface of the receptive film, thereby impairing its use as a transparency.
Embossing in the instance of transparent films is particularly undesirable as the embossing pattern often projects as a gray background on overhead projection screens, thereby reducing image contrast and the readability of the overhead transparency's projected image. Measures utilized to prevent embossing, heretofore have usually involved the use of crosslinking systems, to harden the soft resin binders, normally employed in receptor sheet surface coatings. However, such conventional crosslinking also reduces image bonding and therefore, image adhesion, so that poor quality overhead transparencies are still obtained. This is especially true if a high speed copier is utilized to prepare such imaged transparencies.
Thermoplastic resins having a high glass transition temperature (Tg), have also been utilized to resolve embossing problems. However, the use of such thermoplastic resins has often resulted in the problem of poor toner adhesion. Thus, when utilizing such high glass transition temperature thermoplastic resins, there is often encountered the classical problem of unfavorable tradeoffs, wherein one desirable quality is obtained at the expense of another.
Electrophotographic image processing may often be accompanied by transport problems of the copying media through the electrophotographic copier or printer. Such transport problems are manifested by the occurrence of multiple feeds, jams, or stacking problems in the copiers and printers. These problems are increasingly more pronounced, as copying and printing speeds become faster, often due to static build-up on the surface of the film utilized.
The use of conventional liquid anti-static agents to help provide reliable film transport in electrophotographic copiers and printers, have heretofore often reduced toner adhesion to imaging films, resulting in image loss during the handling or use thereof. Moreover, conventional liquid anti-static agents can often migrate from the surface of such film substrates, so that the occurrence of multiple feeds or jams, can still be relatively commonplace, even when conventional anti-static agents are utilized. Examples of said liquid anti-stats are Cyanamid's Cyastat 609 and SN.
A review of the prior art illustrates the problems in achieving the required balance of properties.
U.S. Pat. No. 4,071,362 pertains to a process for electrophotographic copying and printing in which the media have improved toner receptivity and reduced double feeding. While its objectives have some similarity to those of the present invention, the composition is different and less effective. This prior art utilizes a thermoplastic polymer(s), which is said to provide good toner adhesion. An important difference from the present invention is that it employs no crosslinking polymeric system. Another difference is its use of a non-polymeric anti-stat agent on the side opposite to the imaging side of the film.
U.S. Pat. No. 4,415,626 pertains to electrographic copying and printing media which during the imaging process are less likely to stick to one another or jam in electrographic copier/duplicator equipment. This prior art teaches the preferable use of a single polymer which is hardened by means of a Werner chromium complex. Its anti-static receiving layer does not employ a separate polymer of low Tg to achieve superior toner bonding and also requires a charge control agent.
A study of the related art described above shows that it did not consider the need for a single film product which possesses both superior toner adhesion required by today's high speed copiers and printers and embossing resistance required by slow hot fusing copiers and printers.
In contrast to the related art, the present invention uses a combination of thermoplastic polymer selected to provide superior toner adhesion and thermosetting polymer selected to provide embossing resistance. Thus, the qualities of toner adhesion and embossing resistance can be independently and optimally imparted. Further, the intermingling of the two polymers to form a matrix provides a unique ability to enhance these desired qualities. A further novel feature of the present invention is that it allows for the use of an electrically conductive polymer which may be cross-linkable. Thus, in the present invention two separate and desirable properties can be achieved with a single polymer, namely embossing resistance and conductivity.